Field of the Invention
The present invention relates to catalyst layers for electrochemical cells, in particular, cathode catalyst layers of membrane electrode assemblies for electrochemical fuel cells.
Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly that includes a solid polymer electrolyte membrane disposed between two electrodes. The membrane electrode assembly is typically interposed between two electrically conductive flow field plates to form a fuel cell. These flow field plates act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such flow field plates typically include fluid flow channels to direct the flow of the fuel and oxidant reactant fluids to an anode and a cathode of each of the membrane electrode assemblies, respectively, and to remove excess reactant fluids and reaction products. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of fuel cells are electrically coupled in series to form a fuel cell stack having a desired power output.
The anode and the cathode each contain a layer of anode catalyst and cathode catalyst, respectively. The catalyst may be a metal, an alloy or a supported metal/alloy catalyst, for example, platinum supported on carbon black. The catalyst layer may contain an ion conductive material, such as NAFION® (provided by E. I. du Pont de Nemours and Co.) and/or a binder, such as polytetrafluoroethylene (PTFE). Each electrode further includes an electrically conductive porous substrate, such as carbon fiber paper or carbon cloth, for reactant distribution and/or mechanical support. The thickness of the porous substrate typically ranges from about 50 to about 250 microns. Optionally, the electrodes may include a porous sublayer disposed between the catalyst layer and the substrate. The sublayer usually contains electrically conductive particles, such as carbon particles, and, optionally, a water repellent material for modifying its properties, such as gas diffusion and water management. The catalyst may be coated onto the membrane to form a catalyst-coated membrane (CCM) or coated onto the sublayer or the substrate to form an electrode.
The catalyst is one of the most expensive components in a fuel cell due to the noble metals that are typically used. Such noble metals include platinum and gold, which are often mixed with or alloyed with other metals, such as ruthenium, iridium, cobalt, nickel, molybdenum, palladium, iron, tin, titanium, manganese, cerium, chromium, copper, and tungsten, to enhance preferred reactions and mitigate unwanted side reactions, which are different for the anode and the cathode.
The anode and cathode half-cell reactions in hydrogen gas fuel cells are shown in the following equations:H2→2H++2e−  (1)½O2+2H++2e−→H2O  (2)
On the anode, the primary function is to oxidize hydrogen fuel to form protons and electrons. Depending on the fuel source, the anode catalyst may need to be tolerant to impurities. For example, carbon monoxide poisoning of the anode catalyst often occurs when operating on a reformate-based fuel. To mitigate carbon monoxide poisoning, a platinum alloy catalyst, such as platinum-ruthenium, is preferable on the anode.
On the cathode, the primary function is to reduce oxygen and form water. This reaction is inherently much slower than the anode reaction and, thus, the cathode catalyst loading is typically higher than the anode catalyst loading. One way of enhancing the cathode half-cell reaction is to improve the electrochemical activity and catalyst utilization of the catalyst layer, thereby reducing voltage losses related to catalytic kinetics. For example, U.S. Pat. No. 7,419,740 discloses membrane electrode assemblies having increased activity and improved utilization of the noble metal catalyst. In particular, the membrane electrode assemblies have reaction layers containing a noble metal catalyst supported on carbon and a proton-conducting polymer, wherein the reaction layer on the cathode side comprise at least two sublayers, a first sublayer and a second sublayer, on top of each other, wherein the first sublayer is in direct contact with the polymer electrolyte membrane and contains a noble metal black and a noble metal catalyst supported on carbon and the second sublayer contains a further supported noble metal catalyst. However, using noble metal blacks is not desirable because it is difficult to achieve low catalyst loadings, which is required to reduce cost, and tends to be create denser layers, which creates water management and gas diffusion issues due to the decrease porosity.
At the same time, catalysts in the anode and cathode need to be able to withstand degradation that may occur during fuel cell operation and fuel cell start-up and shutdown. Typical catalyst degradation modes include corrosion of the catalyst support material and platinum dissolution and agglomeration, which leads to a decrease in fuel cell performance due to the decreased platinum surface area. Catalyst degradation is an important issue because it has a detrimental impact on fuel cell lifetime and overall costs. To mitigate corrosion, graphitized carbon supports are preferable over carbon black supports because graphitized carbon supports are more stable and less susceptible to corrosion. According to M. Murthy and N. Sisofo (“Investigation of Degradation Mechanisms Relevant to Automotive Fuel Cells”, 3rd European PEFC Forum, Session B03, File No. B031 (2005)), it is believed that the graphitization process removes surface functional groups, which reduces the extent of carbon corrosion. However, graphitized carbon supports also have a lower surface area, which makes it difficult to homogeneously disperse noble metal catalysts onto graphitized carbon supports. Therefore, catalysts having noble metals dispersed on graphitized carbon supports typically show a lower electrochemical activity and fuel cell performance than catalysts having noble metals dispersed on high surface area supports, such as carbon black.
As a result, there still exists much research in catalyst and catalyst layer designs to improve performance and durability of the catalysts while reducing costs. The present description addresses these issues and provides further related advantages.